Enhancing Defect Tolerance and Phase Stability of High-Bandgap Perovskites via Guanidinium Alloying

The open-circuit voltages (VOC) of hybrid perovskite (HP) solar cells do not increase sufficiently with increasing bandgap (for Eg > 1.70eV). We study the impact of A+ size mismatch induced lattice distortions (in ABX3 structure) on the optoelectronic quality of high-bandgap HPs and find that the highest quality films have high A-site size-mismatch, where large guanidinium (GA) compensates for small Cs to keep the tolerance factor in the range for the perovskite structure. Specifically, we find that 1.84eV bandgap (FA0.33GA0.19Cs0.47)Pb(I0.66Br0.34)3 and 1.75eV bandgap (FA0.58GA0.10Cs0.32)Pb(I0.73Br0.27)3 attain quasi-Fermi level splitting of 1.43eV and 1.35eV, respectively, which is >91% of the Shockley-Queisser limit for both cases. Films of 1.75eV bandgap (FA,GA,Cs)Pb(I,Br)3 are then used to fabricate p-i-n photovoltaic devices that have a VOC of 1.24 V. This VOC is among the highest VOC reported for any HPs with similar bandgap (1.7 to 1.8 eV) and a substantial improvement for the p-i-n architecture, which is desirable for tandems with Si, CIGS, or a low-bandgap HP. Collectively, our results show that non-radiative recombination rates are reduced in (FA,GA,Cs)Pb(I,Br)3 films and prove that FA-GA-Cs alloying is a viable route to attain high VOC in high-bandgap HP solar cells.